Nanotube-based logic driver circuits

ABSTRACT

Nanotube based logic driver circuits. These include pull-up driver circuits, push-pull driver circuits, tristate driver circuits, among others. Under one embodiment, an off-chip driver circuit includes a differential input having first and second signal links, each coupled to a respective one of two differential, on-chip signals. At least one output link is connectable to an off-chip impedance load, and at least one switching element has an input node, an output node, a nanotube channel element, and a control structure disposed in relation to the nanotube channel element to controllably form and unform an electrically conductive channel between said input node and said output node. The input node is coupled to a reference signal and the control structure is coupled to the first and second signal links. The output node is coupled to the output link, and the channel element is sized to carry sufficient current to drive said off-chip impedance load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. §120 to U.S. patent application Ser. No. 11/033,216, filed onJan. 10, 2005 now U.S. Pat. No. 7,164,744, entitled Nanotube-based LogicDriver Circuits, which claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application No. 60/581,074, filed on Jun. 18, 2004,entitled Nonvolatile Carbon Nanotube Logic (NLOGIC) Off Chip Driver,both of which are incorporated herein by reference in their entirety.

This application is related to the following references:

U.S. patent application Ser. No. 10/917,794, now U.S. Pat. No.7,115,960, filed on Aug. 13, 2004, entitled Nanotube-Based SwitchingElements;

U.S. patent application Ser. No. 10/918,085, now U.S. Pat. No.6,990,009, filed on Aug. 13, 2004, entitled Nanotube-Based SwitchingElements With Multiple Controls;

U.S. patent application Ser. No. 10/918,181, now U.S. Pat. No.7,071,023, filed on Aug. 13, 2004, entitled Nanotube Device StructureAnd Methods Of Fabrication;

U.S. patent application Ser. No. 10/917,893, now U.S. Pat. No.7,138,832, filed on Aug. 13, 2004, entitled Nanotube-Based SwitchingElements And Logic Circuits;

U.S. patent application Ser. No. 10/917,606, filed on Aug. 13, 2004,entitled Isolation Structure For Deflectable Nanotube Elements;

U.S. patent application Ser. No. 10/917,932, filed on Aug. 13, 2004,entitled Circuits Made From Nanotube-Based Switching Elements WithMultiple Controls;

U.S. patent application Ser. No. 11/033,087, filed on Jan. 10, 2005,entitled Nanotube-Based Transfer Devices and Related Circuits;

U.S. patent application Ser. No. 11/033,089, filed on Jan. 10, 2005,entitled Integrated Nanotube and Field Effect Switching Device;

U.S. patent application Ser. No. 11/033,213, filed on Jan. 10, 2005,entitled Receiver Circuit Using Nanotube-Based Switches and Transistors;

U.S. patent application Ser. No. 11/033,215, filed on Jan. 10, 2005,entitled Receiver Circuit Using Nanotube-based Switches and Logic;

U.S. patent application Ser. No. 11/032,983, filed on Jan. 10, 2005,entitled Storage Elements Using Nanotube Switching Elements; and

U.S. patent application Ser. No. 11/032,823, now U.S. Pat. No.7,167,026, filed on Jan. 10, 2005, entitled Tri-State Circuit UsingNanotube Switching Elements.

BACKGROUND

1. Technical Field

The present application generally relates to nanotube switching circuitsand in particular to nanotube switching circuits used in driver circuitscapable of driving relatively high capacitive loads.

2. Discussion of Related Art

Digital logic circuits are used in personal computers, portableelectronic devices such as personal organizers and calculators,electronic entertainment devices, and in control circuits forappliances, telephone switching systems, automobiles, aircraft and otheritems of manufacture. Early digital logic was constructed out ofdiscrete switching elements composed of individual bipolar transistors.With the invention of the bipolar integrated circuit, large numbers ofindividual switching elements could be combined on a single siliconsubstrate to create complete digital logic circuits such as inverters,NAND gates, NOR gates, flip-flops, adders, etc. However, the density ofbipolar digital integrated circuits is limited by their high powerconsumption and the ability of packaging technology to dissipate theheat produced while the circuits are operating. The availability ofmetal oxide semiconductor (“MOS”) integrated circuits using field effecttransistor (“FET”) switching elements significantly reduces the powerconsumption of digital logic and enables the construction of the highdensity, complex digital circuits used in current technology. Thedensity and operating speed of MOS digital circuits are still limited bythe need to dissipate the heat produced when the device is operating.

Digital logic integrated circuits constructed from bipolar or MOSdevices do not function correctly under conditions of high heat or heavyradiation. Current digital integrated circuits are normally designed tooperate at temperatures less than 100 degrees centigrade and few operateat temperatures over 200 degrees centigrade. In conventional integratedcircuits, the leakage current of the individual switching elements inthe “off” state increases rapidly with temperature. As leakage currentincreases, the operating temperature of the device rises, the powerconsumed by the circuit increases, and the difficulty of discriminatingthe off state from the on state reduces circuit reliability.Conventional digital logic circuits also short internally when subjectedto heavy radiation because the radiation generates electrical currentsinside the semiconductor material. It is possible to manufactureintegrated circuits with special devices and isolation techniques sothat they remain operational when exposed to heavy radiation, but thehigh cost of these devices limits their availability and practicality.In addition, radiation hardened digital circuits exhibit timingdifferences from their normal counterparts, requiring additional designverification to add radiation protection to an existing design.

Integrated circuits constructed from either bipolar or FET switchingelements are volatile. They only maintain their internal logical statewhile power is applied to the device. When power is removed, theinternal state is lost unless some type of non-volatile memory circuit,such as EEPROM (electrically erasable programmable read-only memory), isadded internal or external to the device to maintain the logical state.Even if non-volatile memory is utilized to maintain the logical state,additional circuitry is necessary to transfer the digital logic state tothe memory before power is lost, and to restore the state of theindividual logic circuits when power is restored to the device.Alternative solutions to avoid losing information in volatile digitalcircuits, such as battery backup, also add cost and complexity todigital designs.

Important characteristics for logic circuits in an electronic device arelow cost, high density, low power, and high speed. Resistance toradiation and the ability to function correctly at elevated temperaturesalso expand the applicability of digital logic. Conventional logicsolutions are limited to silicon substrates, but logic circuits built onother substrates would allow logic devices to be integrated directlyinto many manufactured products in a single step, further reducing cost.

Devices have been proposed which use nanoscopic wires, such assingle-walled carbon nanotubes, to form crossbar junctions to serve asmemory cells. (See WO 01/03208, Nanoscopic Wire-Based Devices, Arrays,and Methods of Their Manufacture; and Thomas Rueckes et al., “CarbonNanotube-Based Nonvolatile Random Access Memory for MolecularComputing,” Science, vol. 289, pp. 94-97, 7 Jul. 2000.) Hereinafterthese devices are called nanotube wire crossbar memories (NTWCMs). Underthese proposals, individual single-walled nanotube wires suspended overother wires define memory cells. Electrical signals are written to oneor both wires to cause them to physically attract or repel relative toone another. Each physical state (i.e., attracted or repelled wires)corresponds to an electrical state. Repelled wires are an open circuitjunction. Attracted wires are a closed state forming a rectifiedjunction. When electrical power is removed from the junction, the wiresretain their physical (and thus electrical) state thereby forming anon-volatile memory cell.

U.S. Patent Publication No. 2003-0021966 discloses, among other things,electromechanical circuits, such as memory cells, in which circuitsinclude a structure having electrically conductive traces and supportsextending from a surface of a substrate. Nanotube ribbons that canelectromechanically deform, or switch are suspended by the supports thatcross the electrically conductive traces. Each ribbon comprises one ormore nanotubes. The ribbons are typically formed from selectivelyremoving material from a layer or matted fabric of nanotubes.

For example, as disclosed in U.S. Patent Publication No. 2003-0021966, ananofabric may be patterned into ribbons, and the ribbons can be used asa component to create non-volatile electromechanical memory cells. Theribbon is electromechanically-deflectable in response to electricalstimulus of control traces and/or the ribbon. The deflected, physicalstate of the ribbon may be made to represent a corresponding informationstate. The deflected, physical state has non-volatile properties,meaning the ribbon retains its physical (and therefore informational)state even if power to the memory cell is removed. As explained in U.S.Patent Publication No. 2003-0124325, three-trace architectures may beused for electromechanical memory cells, in which the two of the tracesare electrodes to control the deflection of the ribbon.

The use of an electromechanical bi-stable device for digital informationstorage has also been suggested (c.f. U.S. Pat. No. 4,979,149:Non-volatile memory device including a micro-mechanical storageelement).

The creation and operation of bi-stable, nano-electro-mechanicalswitches based on carbon nanotubes (including mono-layers constructedthereof) and metal electrodes has been detailed in a previous patentapplication of Nantero, Inc. (U.S. Pat. Nos. 6,574,130, 6,643,165,6,706,402; U.S. patent application Ser. Nos. 09/915,093, 10/033,323,10/033,032, 10/128,117 , 10/341,005, 10/341,055, 10/341,054, 10/341,130,10/776,059, and 10/776,572, the contents of which are herebyincorporated by reference in their entireties).

SUMMARY

The invention provides nanotube based logic driver circuits. Theseinclude pull-up driver circuits, push-pull driver circuits, tristatedriver circuits, among others.

Under one aspect of the invention, an off-chip driver circuit includes adifferential input having a first and second signal links, each coupledto a respective one of two differential, on-chip signals. At least oneoutput link is connectable to an off-chip impedance load, and at leastone switching element has an input node, an output node, a nanotubechannel element, and a control structure disposed in relation to thenanotube channel element to controllably form and unform an electricallyconductive channel between said input node and said output node. Theinput node is coupled to a reference signal and the control structure iscoupled to the first and second signal links. The output node is coupledto the output link, and the nanotube channel element is sized to carrysufficient current to drive said off-chip impedance load.

Under another aspect of the invention, the output link is coupled to apull-up resistive load coupled to a supply voltage.

Under another aspect of the invention, the supply voltage coupled to theresistive load is different than the supply voltage of a chip having theoff-chip driver circuit.

Under another aspect of the invention, the off-chip driver circuitfurther includes a second switching element having an input node, anoutput node, a nanotube channel element, and a control structuredisposed in relation to the nanotube channel element to controllablyform and unform an electrically conductive channel between said inputnode and said output node; and further comprising a second output linkconnectable to a second off-chip impedance load. The input node of thesecond switching element is coupled to a reference signal and thecontrol structure of the second switching element is coupled to thefirst and second signal links. The output node of the second switchingelement is coupled to the second output link, and the nanotube channelelement of the second switching element is sized to carry sufficientcurrent to drive the off-chip impedance load.

Under another aspect of the invention, an off-chip driver circuitincludes a differential input having a first and second signal links,each coupled to a respective one of two differential, on-chip signals.At least one output link is connectable to an off-chip impedance load.At least first and second switching elements each have an input node, anoutput node, a nanotube channel element, and a control structuredisposed in relation to the nanotube channel element to controllablyform and unform an electrically conductive channel between said inputnode and said output node. The input node of the first switching elementis coupled to a first reference signal and input node of the secondswitching element is coupled to a second reference signal. The controlstructure of each of the first and second switching elements is coupledto the first and second signal links and the output node of each of thefirst and second switching elements is coupled to the output link. Thenanotube channel element of each of the first and second switchingelements is sized to carry sufficient current to drive said off-chipimpedance load.

Under another aspect of the invention, an off-chip driver circuitincludes a differential input having a first and second signal links,each coupled to a respective one of two differential, on-chip signals.First and second output links are each connectable to an off-chipimpedance load. The circuit further includes at least first and secondpush-pull drivers, each push-pull driver connected to the first andsecond input links and each connected to a respective one of the firstand second output links. Each push-pull driver includes first and secondswitching elements, each having an input node, an output node, ananotube channel element, and a control structure disposed in relationto the nanotube channel element to controllably form and unform anelectrically conductive channel between said input node and said outputnode. The input node of the first switching element is coupled to afirst reference signal and input node of the second switching element iscoupled to a second reference signal. The control structure of each ofthe first and second switching elements is coupled to the first andsecond signal links and the output node of each of the first and secondswitching elements is coupled to a respective one of the output links.The nanotube channel element of each of the first and second switchingelements is sized to carry sufficient current to drive said off-chipimpedance load.

Under another aspect of the invention, an off-chip tristate drivercircuit includes a differential input having a first and second signallinks, each coupled to a respective one of two differential, on-chipsignals. At least one output link is connectable to an off-chipimpedance load. The circuit includes at least first and second switchingelements, each having an input node, an output node, a nanotube channelelement, and a control structure disposed in relation to the nanotubechannel element to controllably form and unform an electricallyconductive channel between said input node and said output node. Thecontrol structure of each of the first and second switching elements iscoupled to the first and second signal links and the output node of eachof the first and second switching elements is coupled to the outputlink. The nanotube channel element of each of the first and secondswitching elements is sized to carry sufficient current to drive saidoff-chip impedance load. The input node of the first switching elementis coupled to a first reference signal through a first selectionstructure and the input node of the second switching element is coupledto a second reference signal through a second selection structure.

Under another aspect of the invention, the first and second selectionstructures are each switching elements having an input node connected toa respective one of the first and second reference signals, an outputnode connected to a respective input node of the first and secondswitching elements, a nanotube channel element, and a control structuredisposed in relation to the nanotube channel element to controllablyform and unform an electrically conductive channel between said inputnode and said output node, and the control structure is coupled of eachselection structure is coupled to at least one selection signal.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a transverse cross-sectional view of a nanotube in relation toa substrate;

FIG. 2 is a plan view of two separated nanotubes;

FIG. 3 is a plan view of a plurality of nanotubes suspended by contacts;

FIG. 4A is a single rail driver circuit according to certainembodiments;

FIG. 4B is a dual rail driver circuit according to certain embodiments;

FIG. 5 depicts a pre-driver circuit that may be used in conjunction witha driver circuit according to certain embodiments of the invention;

FIGS. 6A-D depict a preferred nanotube switching element incross-section and layout views and in two informational states;

FIGS. 7A-C depict the notation used to describe the nanotube switch andits states;

FIG. 8 depicts a single rail, push pull driver circuit according tocertain embodiments;

FIG. 9 depicts a dual rail, push pull driver circuit according tocertain embodiments;

FIG. 10 depicts a single rail, tristate, push pull driver circuitaccording to certain embodiments; and

FIG. 11 depicts a dual rail, tristate, push pull driver circuitaccording to certain embodiments.

DETAILED DESCRIPTION

Preferred embodiments of the invention arrange nanotube-based switchesinto driver circuits capable of driving relatively large capacitances,such as off chip (OC) capacitances, long or heavily loaded interconnectlines, etc. The circuits may be either dual-rail (differential) orsingle-rail, open carbon nanotube (CNT) output (corresponds to opendrain in an FET), push pull drivers, and tristate push pull drivers, forexample.

Carbon nanotube-based logic circuits, such as those circuits disclosedin the incorporated, related references, may be required to drive arelatively large capacitance, 10 pf for example, when driving off-chip,or when driving heavy loads on-chip. Nanotube-based logic circuits usingporous carbon nanotube fabrics to construct nanotube channel elementsare typically low capacitance circuits, e.g., 30 aF (0.030 fF) input andoutput capacitances, and can typically drive only relatively lowcapacitances. Preferably, a logic family includes circuits that candrive large capacitive loads, such as 10 pf.

FIG. 1 depicts in transverse cross-sectional view of a single nanotube15 (e.g., carbon nanotube) of diameter 2R (typically 0.6 to 1.5 nm indiameter) and height h above a conductive region 20, where theconductive region is typically a control (or set) or a releaseelectrode, such as disclosed in the incorporated, related references.

FIG. 2 illustrates a plan view of two nanotube fibers 15 of diameter 2Rand a void region XVOID between fibers in the porous nanotube fabriclayer.

FIG. 3 illustrates a plan view of multiple carbon nanotubes, labeled 15a-n of length LSUSP. The CNTs are suspended between NT contactconductors 40 to form the nanotube channel elements of exemplary carbonnanotube switches, such as those disclosed in the incorporated, relatedreferences. In this figure, the nanotubes are depicted as if they arealigned in parallel, but in preferred embodiments such alignment is notnecessary and in many embodiments such strict parallel alignment is notexpected. As described in the incorporated references, LSUSP for a modemimplementation may be about 325 nm (0.325 μm), for example.

Present nanotube fabric densities may be approximately 10 nanotubes in a200×200 nm region. A 325×325 nm region comprising a suspended lengthLSUSP=325 nm and width=325 nm contains approximately 15 NTs, forexample. XVOID may be calculated using the equation 15(2R+XVOID)=325 nm,where 2R=1.5 nm, for example. XVOID is approximately equal to 20 nm, forexample. For 2R=1.5 nm, and XVOID=20 nm, the nanotube fabric density isapproximately 7% NTs with a 93% void region. In operation, the NT fibersare in contact with the insulator layer of a control or releaseelectrodes (with a conductive element under the insulator layer). Thecontrol or release electrode conductor approximates ground plane 20shown in FIG. 1.

Capacitance between the nanotube and the control or release electrodesis contributed by an approximately 65 nm portion of NT fiber length inclose proximity to the electrodes. The insulator thickness is assumed tobe approximately 15 nm (h=15 nm as shown in FIG. 1), and the dielectricis assumed to be SiO₂ with relative dielectric constant of approximately4. The capacitance per nanotube is calculated using the followingequation:C=πεl/cos h ⁻¹(1+h/R), where cos h ⁻¹ =πγl/ln[(1+h/R)+{(1+h/R)²−1}^(0.5)]as given in Ramo, S. and Whinnery, J. R., Fields and Waves in ModemRadio, John Wiley and Sons, 1958 138-14. For 15 nanotubes in parallel,the nanotube electrostatic switch input capacitance is 30 aF (0.030 fF).Output capacitance and local wiring capacitance is estimated as also 30aF.

The resistance of a nanotube nonvolatile electrostatic switch consistsof a conductor to nanotube contact resistance Rc in series with ananotube to output electrode conductor contact resistance (when theswitch is in the “ON” state) of RSW. Nanotube switches (e.g.,non-volatile) are typically symmetrical, and Rc consists of two contactsin parallel. RSW also uses two contacts in parallel. Each conductor tonanotube contact is approximately 20,000 Ohms, for example, and eachswitching contact is approximately 30,000 Ohms, for example. The totalresistance between the nanotube contacts and the output contacts isapproximately 25,000 Ohms per nanotube, for example. The resistance isdominated by contact resistance, so that the nanotube fiber resistanceis negligible. For 15 parallel nanotubes, the nanotube resistance isapproximately 1,700 ohms.

FIG. 4A illustrates a nanotube nonvolatile off chip driver (OCD) 50. Theterm off chip driver is used to refer to a circuit driving a heavycapacitive load, such as 10 pF for example, whether the load is on thesame chip or on another chip. Driver 50 includes nanotube switchingelement 65 (more below) connected to output terminal 75 and drives acapacitive load COUT, and a pull-up resistor RLOAD 70 (COUT and RLOADare not part of output driver 50) connected to power supply voltageVDDQ. The driver 50 is coupled to a logic circuit (a receiver, forexample). Nanotube switching element 65 is driven by the outputs of apredriver stage at terminals X and Y, with true and complementary logicinputs AT and AC, respectively. The output 75 of nanotube switchingelement 65 is connected to capacitive load COUT, pull-up resistor RLOAD,and a logic input. The nanotube channel element (more below) isconnected to ground as a reference via a signal electrode (more belowwhen describing an exemplary nanotube switching element). Preferredembodiments form switching element 65 such that the nanotube channelelement has a number of nanotubes to provide a resistance RNT betweenoutput and ground of approximately 60 Ohms. A switch with 15 nanotubesin parallel has a resistance of 1,700 Ohms, an input capacitance ofapproximately 30 aF (0.030 fF), and occupies a width of 302.5 nm(approximately 0.30 um). A switch with 425 parallel nanotubes has aresistance of approximately 60 ohms, an input capacitance ofapproximately 850 aF (0.85 fF), and occupies a width of 9,117.5 nm(approximately 9.12 um). The capacitance load COUT is approximately 10pF, for example. The pull-up resistor RLOAD must be much larger thanRNT, and is selected as 600 ohms, for example.

In operation, if input AT is at a positive voltage and AC is at ground,switching element 65 is turned “ON” (CLOSED), and output 75 is connectedto ground through a 60 Ohm resistance (of the switch 65 ) dischargingcapacitor COUT. If COUT=10 pF, the discharge time constant RNTCOUT isapproximately 600 ps, and the output fall time is less than 1.5 ns. Notethat as long as output 75 is held at ground, a dc current will flow frompower source VDDQ through resistor 70 and driver 65 to ground. The dccurrent is a consequence of the open-CNT output design, corresponding toan open-drain FET design and will be discussed further below. Note thatoutput drivers are often powered by a dedicated power bus at a voltagereferred to as VDDQ and other circuits in the chip by a separate powerbus referred to as VDD in order to minimize on-chip-noise when switchinglarge output driver circuits. VDDQ may be the same voltage as VDD or maybe at a higher or lower voltage than VDD. Examples of VDD and VDDQ andare 5, 3.3, 2.5, and 1.8 volts.

In operation, if input AT is at zero volts and AC is a positive voltage,switch 65 is turned “OFF” (OPEN), and node 75 voltage rises to VDDQ asresistor RLOAD charges capacitor COUT. The charge time constantRLOADCOUT is 6 ns, and the output rise time is less than 15 ns. No dccurrent flows when output node 75 is in the open state.

The voltage applied to driver input terminals X and Y does not have tobe equal to VDDQ. The driver input voltages applied to terminals X and Ymay be 1.5 volts, for example, and VDDQ may be 3.3 volts, for example.Driver 50 may be used to drive a broad range of VDDQ voltage levels,from less than 1 volt to greater than 5 volts, for example.

FIG. 4B illustrates a dual-rail output driver 80. The circuit 80includes nanotube switching elements 85 and 90, interconnected as shown,and has dual-rail (differential) inputs X and Y and dual-rail(differential) outputs 95 and 95′. Each output drives a capacitance loadCOUT, a resistor RLOAD, and a logic input. Each switching element 85 and90 is sized as described above to provide appropriate drive (e.g., wideenough to have 425 nanotubes in parallel), and has an “ON” resistance ofabout 60 Ohms. The input capacitance at inputs X and Y is approximately0.85 fF. The operation of nanotube switching elements 85 and 90 shown inFIG. 4B is the same as the operation of nanotube switch 65 shown in FIG.4A and described earlier.

Output driver 50 and output driver 80 each require a pre-driveramplification stage. The output drivers use nanotube switches with 425parallel nanotubes with an input capacitance of 0.85 fF, while logic inthis example typically uses nanotube switches with 15 parallelnanotubes, having an input capacitance of 0.030 fF. A pre-driver is usedto drive output drivers 50 and 80. A pre-driver consists of a chain(several stages) of nanotube inverters of increasing size. Thepre-driver uses enough stages so that the last pre-driver stage isstrong enough to drive the input capacitance of the nanotube outputdriver. It is well known (G. E. Sobelman, “Advanced Digital CircuitDesign”, EE course printed notes, copyright 2001”, pages 9-15, and H. B.Bakoglu, “Circuits, interconnections, and Packaging for VLSI”,Addison-Wesley Publishing Co., 1990, pages 172-173) that in order tominimize the total delay along a pre-driver path, each pre-driver stageshould carry approximately the same load, typically expressed as thesame COUT/CIN capacitance ratio at each stage, with COUT/CIN in the 3 to5 range.

FIG. 5 illustrates nanotube-based three-stage pre-driver 100. Each stageis composed of a dual-rail (differential) nanotube nonvolatile inverterlogic stage with input and output loads indicated as C1-C4. Theoperation of dual-rail nanotube inverters is described in some of theincorporated, related references. The outputs X and Y of pre-driver 100connect to inputs X and Y of driver 50 in FIG. 4A and driver 80 of FIG.4B. Capacitance C4 is 0.85 fF, i.e., the input capacitance of OCD 50 orOCD 80. Capacitance C1 is the input capacitance 0.030 fF of a 15 NTnanotube switch. Selecting a C_(OUT)/C_(IN) design ratio of 3, the NTswitches in pre-driver 100 stages 1-3 and OCD 50 or 80 are summarized inTable 1.

TABLE 1 Stage 1 Stage 2 Stage 3 OCD # NTs in parallel 15 45 135 425    # NTs Ratio 1 3 9 28    (relative to stage 1) C_(IN) (fF) 0.03 0.09 0.270.85  C_(OUT) (fF) 0.09 0.27 0.85 10,000*      NT Switch L_(SUSP) (μm)0.325 0.325 0.325 0.325 NT Switch Width (μm) 0.325 0.975 2.925 8.775 *10pf load driven by the OCD using a load pull-up approach shown in FIG.4A&B

Table 1 shows the NT switch characteristics used at each of the threepre-amplifier 100 stages, stage 1, stage 2, and stage 3. Table 1 alsoshows the characteristics of the OCD NT output driver (switch) 50 and80. The load pull-up approach enables an nanotube switching elementbased OCD to discharge a large 10 pF capacitance, and therefore to drivea large capacitive load directly from an NT switch. However, the falltime is much faster than the rise time (approximately 1/10th) and dccurrent flows when the driver output node is at ground.

FIGS. 6A-D depict a preferred nanotube switching element 600 incross-section and layout views and in two informational states. Theseswitches may be used for switches 65, 85 or 90 of FIGS. 4A-B. A moredetailed description of these switches may be found in the related casesidentified and incorporated above. A brief description follows here forconvenience.

FIG. 6A is a cross sectional view of a preferred nanotube switchingelement 100. Nanotube switching element includes a lower portion havingan insulating layer 117, control electrode 111, output electrodes 113c,d. Nanotube switching element further includes an upper portion havingrelease electrode 112, output electrodes 113 a,b, and signal electrodes114 a,b. A nanotube channel element 115 is positioned between and heldby the upper and lower portions.

Release electrode 112 is made of conductive material and is separatedfrom nanotube channel element 115 by an insulating material 119. Thechannel element 115 is separated from the facing surface of insulator119 by a gap height G102.

Output electrodes 113 a,b are made of conductive material and areseparated from nanotube channel element 115 by insulating material 119.

Output electrodes 113 c,d are likewise made of conductive material andare separated from nanotube channel element 115 by a gap height G103.Notice that the output electrodes 113 c,d are not covered by insulator.

Control electrode 111 is made of conductive material and is separatedfrom nanotube channel element 115 by an insulating layer (or film) 118.The channel element 115 is separated from the facing surface ofinsulator 118 by a gap height G104.

Signal electrodes 114 a,b each contact the nanotube channel element 115and can therefore supply whatever signal is on the signal electrode tothe channel element 115. This signal may be a fixed reference signal(e.g., VDD or Ground) or varying (e.g., a Boolean discrete value signalthat can change). Only one of the electrodes 114 a,b need be connected,but both may be used to reduce effective resistance.

Nanotube channel element 115 is a lithographically-defined article madefrom a porous fabric of nanotubes (more below). It is electricallyconnected to signal electrodes 114 a,b. The electrodes 114 a,b andsupport 116 pinch or hold the channel element 115 at either end, and itis suspended in the middle in spaced relation to the output electrodes113 a-d and the control electrode 111 and release electrode 112. Thespaced relationship is defined by the gap heights G102-G104 identifiedabove. For certain embodiments, the length of the suspended portion ofchannel element 115 is about 300 to 350 nm.

Under certain embodiments the gaps G103, G104, G102 are in the range of5-30 nm. The dielectric on terminals 112, 111, and 113 a and 113 b arein the range of 5-30 nm, for example. The carbon nanotube fabric densityis approximately 10 nanotubes per 0.2×0.2 um area, for example. Thesuspended length of the nanotube channel element is in the range of 300to 350 nm, for example. The suspended length to gap ratio is about 5 to15 to 1 for non-volatile devices, and less than 5 for volatileoperation, for example.

FIG. 6B is a plan view or layout of nanotube switching element 100. Asshown in this figure, electrodes 113 b,d are electrically connected asdepicted by the notation ‘X’ and item 102. Likewise electrodes 113 a,care connected as depicted by the ‘X’. In preferred embodiments theelectrodes are further connected by connection 120. All of the outputelectrodes collectively form an output node 113 of the switching element100.

Under preferred embodiments, the nanotube switching element 100 of FIGS.6A and 6B operates as shown in FIGS. 6C and D. Specifically, nanotubeswitching element 100 is in an OPEN (OFF) state when nanotube channelelement is in position 122 of FIG. 6C. In such state, the channelelement 115 is drawn into mechanical contact with dielectric layer 119via electrostatic forces created by the potential difference betweenelectrode 112 and channel element 115. Output electrodes 113 a,b are inmechanical contact (but not electrical contact) with channel element115. Nanotube switching element 100 is in a CLOSED (ON) state whenchannel element 115 is elongated to position 124 as illustrated in FIG.6D. In such state, the channel element 115 is drawn into mechanicalcontact with dielectric layer 118 via electrostatic forces created bythe potential difference between electrode 111 and channel element 115.Output electrodes 113 c,d are in mechanical contact and electricalcontact with channel element 115 at regions 126. Consequently, whenchannel element 115 is in position 124, signal electrodes 114 a and 114b are electrically connected with output terminals 113 c,d via channelelement 115, and the signal on electrodes 114 a,b may be transferred viathe channel (including channel element 115) to the output electrodes 113c,d.

By properly tailoring the geometry of nanotube switching element 100,the nanotube switching element 100 may be made to behave as anon-volatile or a volatile switching element. By way of example, thedevice state of FIG. 6D may be made to be non-volatile by properselection of the length of the channel element relative to the gap G104.(The length and gap are two parameters in the restoring force of theelongated, deflected channel element 115.) Length to gap ratios ofgreater than 5 and less than 15 are preferred for non-volatile device;length to gap ratios of less than 5 are preferred for volatile devices.

The nanotube switching element 101 operates in the following way. Ifsignal electrode 114 and control electrode 111 (or 112) have a potentialdifference that is sufficiently large (via respective signals on theelectrodes), the relationship of signals will create an electrostaticforce that is sufficiently large to cause the suspended, nanotubechannel element 115 to deflect into mechanical contact with electrode111 (or 112). (This aspect of operation is described in the incorporatedpatent references.) This deflection is depicted in FIGS. 6D (and 6C).The attractive force stretches and deflects the nanotube fabric ofchannel element 115 until it contacts the insulated region 118 of theelectrode 111. The nanotube channel element is thereby strained, andthere is a restoring tensil force, dependent on the geometricalrelationship of the circuit, among other things.

By using appropriate geometries of components, the switching element 100then attains the closed, conductive state of FIG. 6D in which thenanotube channel 115 mechanically contacts the control electrode 111 andalso output electrode 113 c,d. Since the control electrode 111 iscovered with insulator 118 any signal on electrode 114 is transferredfrom the electrode 114 to the output electrode 113 via the nanotubechannel element 115. The signal on electrode 114 may be a varyingsignal, a fixed signal, a reference signal, a power supply line, orground line. The channel formation is controlled via the signal appliedto the electrode 111 (or 112). Specifically the signal applied tocontrol electrode 111 needs to be sufficiently different in relation tothe signal on electrode 114 to create the electrostatic force to deflectthe nanotube channel element to cause the channel element 115 to deflectand to form the channel between electrode 114 and output electrode 113,such that switching element 100 is in the CLOSED (ON) state.

In contrast, if the relationship of signals on the electrode 114 andcontrol electrode 111 is insufficiently different, then the nanotubechannel element 115 is not deflected and no conductive channel is formedto the output electrode 113. Instead, the channel element 115 isattracted to and physically contacts the insulation layer on releaseelectrode 112. This OPEN (OFF) state is shown in FIG. 6C. The nanotubechannel element 115 has the signal from electrode 114 but this signal isnot transferred to the output node 113. Instead, the state of the outputnode 113 depends on whatever circuitry it is connected to and the stateof such circuitry. The state of output node 113 in this regard isindependent of channel element voltage from signal electrode 114 andnanotube channel element 115 when the switching element 100 is in theOPEN (OFF) state.

If the voltage difference between the control electrode 111 (or 112) andthe channel element 115 is removed, the channel element 115 returns tothe non-elongated state (see FIG. 6A) if the switching element 100 isdesigned to operate in the volatile mode, and the electrical connectionor path between the electrode 115 to the output node 113 is opened.

Preferably, if the switching element 100 is designed to operate in thenon-volatile mode, the channel element is not operated in a manner toattain the state of FIG. 6A. Instead, the electrodes 111 and 112 areexpected to be operated so that the channel element 115 will either bein the state of FIG. 6C or 6D.

The output node 113 is constructed to include an isolation structure inwhich the operation of the channel element 115 and thereby the formationof the channel is invariant to the state of the output node 113. Sincein the preferred embodiment the channel element is electromechanicallydeflectable in response to electrostatically attractive forces, afloating output node 113 in principle could have any potential.Consequently, the potential on an output node may be sufficientlydifferent in relation to the state of the channel element 115 that itwould cause deflection of the channel element 115 and disturb theoperation of the switching element 100 and its channel formation; thatis, the channel formation would depend on the state of an unknownfloating node. In the preferred embodiment this problem is addressedwith an output node that includes an isolation structure to prevent suchdisturbances from being caused.

Specifically, the nanotube channel element 115 is disposed between twooppositely disposed electrodes 113 b,d (and also 113 a,c) of equalpotential. Consequently, there are equal but opposing electrostaticforces that result from the voltage on the output node. Because of theequal and opposing electrostatic forces, the state of output node 113cannot cause the nanotube channel element 115 to deflect regardless ofthe voltages on output node 113 and nanotube channel element 115. Thus,the operation and formation of the channel is made invariant to thestate of the output node.

Under certain embodiments of the invention, the nanotube switchingelement 100 of FIG. 6A may be used as pull-up and pull-down devices toform power-efficient circuits. Unlike MOS and other forms of circuits,the pull-up and pull down devices may be identical devices and need nothave different sizes or materials. To facilitate the description of suchcircuits and to avoid the complexity of the layout and physical diagramsof FIGS. 6A-D, a schematic representation has been developed to depictthe switching elements.

FIG. 7A is a schematic representation of a nanotube switching element600 of FIG. 6A. The nodes use the same reference numerals.

FIGS. 7B-C depict a nanotube channel element 100 when its signalelectrodes is tied to VDD, and its states of operation. For example,FIG. 7B is a schematic representation of the nanotube switching elementin the OPEN (OFF) state illustrated in FIG. 6C, in which signal node 114and the nanotube channel element 115 are at ground, the controlelectrode 111 is at ground, and the release electrode 112 is at VDD. Thenanotube channel element is not in electrical contact with output node113. FIG. 7C is a schematic representation of the nanotube switchingelement in the CLOSED (ON) state illustrated in FIG. 6D. In this case,signal node 114 and the nanotube channel element 115 are at ground, thecontrol electrode 111 is at VDD, and the release electrode 112 is atground. The nanotube channel element is deflected into mechanical andelectrical contact with the output node 113. Moreover, if as describedabove, geometries are selected appropriately, the contact will benon-volatile as a result of the Van der Waals forces between the channelelement and the uninsulated, output electrode.) The state of electricalcontact is depicted by the short black line 204 representing thenanotube channel element contacting the output terminal 113. Thisresults in the output node 113 assuming the same signal (i.e., VDD) asthe nanotube channel element 115 and signal node 114.

As described above, certain embodiments use a nanotube driver schemewhere the nanotube driver has the following two states: for a low outputvoltage, the nanotube driver is activated, and the nanotube driveroutput is connected to ground through the nanotube fabric dischargingthe large (10 pf) output capacitor. Also, a dc current flows fromvoltage source VDDQ through a pullup resistor and nanotube pulldowndevice to ground for as long as the driver output connected to ground.For a high output voltage, the nanotube driver output is in the openstate, cannot discharge the output capacitive load, no dc current flowsto ground, and the driver output node does not determine the finaloutput voltage. A resistor that is part of the receiving logic and isconnected to a power supply such as VDDQ, for example, charges the largecapacitor, thereby setting the high voltage output state. In thisapproach, the power supply voltage used for the nanotube internal logicmay be different than the voltage swing of the output driver, supportingcommunication between chips (or embedded functions within a chip)operating at different voltages. The size of the nanotube output driveris determined by the nanotube resistance in the “ON” state, which may bein the 50 to 70 ohm range, for example. The input capacitance of thenanotube output driver is substantially larger than the capacitance oftypical on-chip nanotube logic circuits and is driven by a pre-driverstage. A pre-driver consists of a chain (several stages) of nanotubeinverters of increasing size. The pre-driver uses enough stages suchthat the last pre-driver inverter stage is strong enough to drive theinput capacitance of the nanotube output driver as previously discussedwith respect to FIG. 5 and table 1.

The single rail nanotube open-output driver 50 illustrated in FIG. 4Aoccupies a relatively small area in the output region of a chip or anembedded macro, however, single rail nanotube open-output driver 50exhibits asymmetrical rise and fall times (fall times approximately 1/10of rise time), and dissipates standby power because dc current flowsthrough RLOAD and nanotube contact resistance when the driver outputelectrode is at ground.

FIG. 8 illustrates nanotube push pull driver 800 in which output loadresistor RLOAD 70 has been replaced by an output device 65′. Outputdevice 65′ is the same as output device 65 of FIG. 4A, with the nanotubechannel element connected to VDDQ instead of ground. Inputs X and Y aretrue and complement logic inputs similar to inputs X and Y in FIG. 4A.Output electrode 75′ of output device 65′ is connected to (dotted with)output electrode 75 of device 65 to drive common output 850 connected toparasitic capacitive load COUT and a logic function. Driver 800 outputnode 850 has the same rise time and fall time (approximately 1.5 ns, forexample) and does not dissipate standby power when output node 850 is atground or at VDDQ.

The dual rail nanotube open-output driver 80 illustrated in FIG. 4Boccupies a relatively small area in the output region of a chip or anembedded macro; however, dual rail nanotube driver 80 exhibitsasymmetrical rise and fall times and standby power dissipation whenoutput electrode 95 or output electrode 95′ is at ground.

FIG. 9 illustrates a dual rail nanotube push pull driver in which outputresistors RLOAD have eliminated, and dual rail open-output driver 80 hasbeen replaced with dual rail nanotube push pull driver 900. Inputs X andY are true and complement logic inputs similar to inputs X and Y in FIG.4B. Dual rail output driver 900 is formed by using two single rail pushpull drivers 910 and 920 interconnected as illustrated in FIG. 9. Theoperation of single rail push pull drivers 910 and 920 are similar tothe operation of single rail push pull driver 800. Single rail push pulldriver 910 output node 950 drives a first output network with capacitiveoutput load COUT and a logic circuit input connection, and single railpush pull driver 920 output node 950′ drives a second output networkwith a capacitive output load COUT and a logic circuit input connection.Dual rail nanotube push pull driver 900 accepts true and complementlogic inputs and provides true and complement logic outputs on outputnodes 950 and 950′. Driver output nodes 950 and 950′ have the same riseand fall time (approximately 1.5 ns, for example) and do not dissipatestandby power when output nodes 950 and 950′ are at ground or at VDDQ.

Logic networks may be multiplexed, that is, more than one logic functionmay drive a network. For example, several logic functions, such as twoprocessors, may share an output bus and drive the bus at differenttimes. Only one logic function may drive a shared network at any pointin time. The output driver associated with the active function isactivated, and all other drivers connected to the output network areinactive, and are typically in a tristate mode. In a tristate mode, theoutputs of unselected drivers are electrically disconnected from powersupply and ground, and the voltages on the unselected driver outputnodes (connected to the common network) are determined by the activatedoutput driver controlling the bus voltage.

FIG. 10 illustrates single rail nanotube tristate push pull driver 1000.Tristate driver 1000 consists of tristate circuit 1010, tristate circuit1020, and single rail push pull driver 1030. The operation of singlerail push pull driver 1030 is similar to the operation of single railpush driver 800 illustrated in FIG. 8. Inputs X and Y are true andcomplement logic inputs similar to inputs X and Y in FIG. 8. Tristatepush pull driver output 1060 corresponds to output 850 in FIG. 8.Tristate device 1010 has the nanotube channel element connected to powersupply VDDQ, the control gate connected to complement logic input SC,the release gate connected to true logic input ST, and the output node1040 connected to a first nanotube channel element of push pull driver1030. Tristate device 1020 has the nanotube channel element connected toground (GND), the control gate connected to true logic input ST, therelease gate connected to complement logic input SC, and the output node1050 connected to a second nanotube channel element of push pull driver1030.

In operation, if single rail nanotube tristate push pull driver is inthe active state, then tristate devices 1010 is activated such thatfirst nanotube channel element of driver 1030 is connected to voltagesource VDDQ, and tristate device 1020 is activated such that secondnanotube channel element of driver 1030 is connected to ground. That is,a positive voltage is applied to terminals ST and ground is applied toterminals SC. When tristate devices 1010 and 1020 are in the active (orON) state, nanotube push pull driver 1030 is active, with output 1060controlling the state of the corresponding logic network. The resistancepath between VDDQ and output node 1060 when driving node 1060 to apositive voltage is the series resistance of tristate device 1010nanotube channel element plus the series resistance of the first channelelement of nanotube push pull driver 1030 plus associated interconnectresistances. The resistance path between ground (GND) and output node1060 when driving node 1060 to ground is the series resistance oftristate device 1020 nanotube channel element plus the series.resistance of the second channel element of nanotube push pull driver1030 plus associated interconnect resistances. The sizes of devicesnanotube devices 1010, 1020, and those of driver 1030 are adjusted asdescribed above with respect to the descriptions associated with FIG. 4and table 1.

FIG. 11 illustrates dual rail nanotube tristate push pull driver 1100.Tristate driver 1100 consists of tristate device 1110, tristate device1120, single rail push pull driver 1130 forming single rail tristatepush pull driver subsection 1135 with output connected to logic network1140, and tristate device 1150, tristate device 1160, and push pulldriver 1170 forming single rail tristate push pull driver subsection1175 with output connected to logic network 1140′, all interconnected asillustrated in FIG. 11. The operation of single rail tristate push pulldriver subsections 1135 and 1175 are similar to the operation of singlerail tristate push pull driver 1000 shown in FIG. 10. Single railtristate push pull driver subsection 1135 drives a first logic network1140 with capacitive output load COUT and a logic circuit inputconnection, and single rail tristate push pull driver subsection 1175drives a second logic network 1140′ with a capacitive output load COUTand logic circuit input connection. Dual rail nanotube tristate driver1100 accepts true and complement logic inputs X and Y and true tristatelogic input ST and complement logic input SC and provides true andcomplement logic output on first logic network 1140 and second outputnetwork 1140′ when in the active (non-tristate mode) with rise and falltimes of 1.5 ns, for example and no standby power when output nodes 1140and 1140′ are at ground or VDDQ. Dual rail nanotube tristate driver 1100is isolated first logic network 1140 and second logic network 1140′ whenthe tristate mode is activated.

Nanotube-based logic may be used in conjunction with and in the absenceof diodes, resistors and transistors or as part of or a replacement toCMOS, biCMOS, bipolar and other transistor level technologies. Theinterconnect wiring used to interconnect the nanotube device terminalsmay be conventional wiring such as AlCu, W, or Cu wiring withappropriate insulating layers such as SiO2, polyimide, etc, or may besingle or multi-wall nanotubes used for wiring.

The inventors envision additional configurations of volatile andnonvolatile or mixed nanoelectromechanical designs depending upon thespecific application, speed, power requirements and density desired.Additionally the inventors foresee the use of multiwalled carbonnanotubes or nanowires as the switching element of contact points withinthe switch. As the technology node decreases in size from 90 nm to 65 nmand below down to the size of individual nanotubes or nanowires theinventors foresee adapting the basic electromechanical switchingelements and their operation to a generation of nanoscale devices withscaleable performance characteristics concomitant with such sizereduction.

The nanotube switching element of preferred embodiments utilizesmultiple controls for the formation and unformation of the channel. Insome embodiments, the device is sized to create a non-volatile deviceand one of the electrodes may be used to form a channel and the othermay be used to unform a channel. The electrodes may be used asdifferential dual-rail inputs. Alternatively they may be set and used atdifferent times. For example, the control electrode may be used in theform of a clock signal, or the release electrode may be used as a formof clocking signal. Also, the control electrode and release electrodemay be placed at the same voltage, for example, such that the state ofthe nanotube cannot be disturbed by noise sources such as voltage spikeson adjacent wiring nodes.

A FIG. 1 device may be designed to operate as a volatile or non-volatiledevice. In the case of a volatile device, the mechanical restoring forcedue to nanotube elongation is stronger than the van der Waals retainingforce, and the nanotube mechanical contact with a control or releaseelectrode insulator is broken when the electrical field is removed.Typically, nanotube geometrical factors such as suspended length to gapratios of less than 5 to 1 are used for volatile devices. In the case ofa non-volatile device, the mechanical restoring force due to nanotubeelongation is weaker than the van der Waals retaining force, and thenanotube mechanical contact with a control or release electrodeinsulator remains un-broken when the electric field is removed.Typically, nanotube geometrical factors such as suspended length to gapratios of greater than 5 to 1 and less than 15 to 1 are used fornon-volatile devices. An applied electrical field generating anelectromechanical force is required to change the state of the nanotubedevice. Van der Waals forces between nanotubes and metals and insulatorsare a function of the material used in the fabrication nanotubeswitches. By way of example, these include insulators such as silicondioxide and silicon nitride, metals such as tungsten, aluminum, copper,nickel, palladium, and semiconductors such as silicon. For the samesurface area, forces will vary by less than 5% for some combinations ofmaterials, or may exceed 2× for other combinations of materials, so thatthe volatile and non-volatile operation is determined by geometricalfactors such as suspended length and gap dimensions and materialsselected. It is, however, possible to design devices by choosing bothgeometrical size and materials that exhibit stronger or weaker van derWaals forces. By way of example, nanotube suspended length and gapheight and fabric layer density, control electrode length, width, anddielectric layer thickness may be varied. Output electrode size andspacing to nanotube may be varied as well. Also, a layer specificallydesigned to increase van der Waals forces (not shown) may be addedduring the fabrication nanotube switching element 100 illustrated inFIG. 1. For example, a thin (5 to 10 nm, for example) layer of metal(not electrically connected), semiconductor (not electricallyconnected), or insulating material may be added (not shown) on theinsulator layer associated with control electrode 111 or releaseelectrode 112 that increases the van der Waals retaining force withoutsubstantial changes to device structure for better non-volatileoperation. In this way, both geometrical sizing and material selectionare used to optimize device operation, in this example to optimizenon-volatile operation.

The 4-terminal devices of FIG. 1 may also be constructed with a nanotubelength to gap size ratio of less than 5 to create a volatile device.This 4-terminal volatile device may also be operated as dual-rail,differential logic but will not preserve the logic state when the powerto the circuit is interrupted. A 4-terminal volatile device may beoperated as a 3-terminal volatile device if the release electrode isconnected to the nanotube channel element through a low resistanceelectrical path such as a metallization layer. For example, releaseterminal 112 may be electrically connected to nanotube signal electrode114. This allows single-rail volatile logic, dual-rail volatile logic,and dual-rail non-volatile logic to be mixed on a single substrate usingnanotube switching devices designed for non-volatile operation, andnanotube switching devices designed for volatile operation.

In a complementary circuit such as an inverter using two nanotubeswitching elements with connected output terminals, there can bemomentary current flow between power supply and ground in the invertercircuit as the inverter changes from one logic state to another logicstate. In CMOS, this occurs when both PFET and NFET are momentarily ON,both conducting during logic state transition and is sometimes referredto as “shoot-through” current. In the case of electromechanicalinverters, a momentary current may occur during change of logic state ifthe nanotube fabric of a first nanotube switch makes conductive contactwith the first output structure before the nanotube fabric of a secondnanotube switch releases conductive contact with the second outputstructure. If, however, the first nanotube switch breaks contact betweenthe first nanotube fabric and the first output electrode before thesecond nanotube switch makes contact between the second nanotube fabricand the second output electrode, then a break-before-make inverteroperation occurs and “shoot-through” current is minimized or eliminated.Electromechanical devices that favor break-before-make operation may bedesigned with different gap heights above and below the nanotubeswitching element, for example, such that forces exerted on the nanotubeswitching element by control and release electrodes are different;and/or travel distance for the nanotube switching element are differentin one direction than another; and/or materials are selected (and/oradded) to increase the van der Waals forces in one switching directionand weakening van der Waals forces in the opposite direction.

By way of example, nanotube switching element 100 illustrated in FIG. 6Amay be designed such that gap G102 is substantially smaller (50%smaller, for example) than gap G104. Also, gap G103 is made bigger suchthat nanotube element 115 contact is delayed when switching. Also,dielectric thicknesses and dielectric constants may be different suchthat for the same applied voltage differences, the electric fieldbetween release electrode 112 and nanotube element 115 is stronger thanthe electric field between control electrode 111 and nanotube element115, for example, to more quickly disconnect nanotube element 115 fromoutput terminals 113 c and 113 d. Output electrodes 113 c and 113 d maybe designed to have a small radius and therefore a smaller contact areain a region of contact with nanotube element 115 compared with the size(area) of contact between nanotube element 115 and the insulator oncontrol terminal 111 to facilitate release of contact between nanotubeelement 115 and output electrodes 113 c and 113 d. The material used forelectrodes 113 c and 113 d may be selected to have weaker van der Waalsforces respect to nanotube element 115 than the van der Waals forcesbetween nanotube element 115 and the insulator on release electrode 112,for example. These, and other approaches, may be used to design ananotube switching element that favors make-before-break operation thusminimizing or eliminating “shoot-through” current as circuits such asinverters switch from one logic state to another.

The material used in the fabrication of the electrodes and contacts usedin the nanotube switches is dependent upon the specific application,i.e. there is no specific metal necessary for the operation of thepresent invention.

Nanotubes can be functionalized with planar conjugated hydrocarbons suchas pyrenes which may then aid in enhancing the internal adhesion betweennanotubes within the ribbons. The surface of the nanotubes can bederivatized to create a more hydrophobic or hydrophilic environment topromote better adhesion of the nanotube fabric to the underlyingelectrode surface. Specifically, functionalization of a wafer/substratesurface involves “derivitizing” the surface of the substrate. Forexample, one could chemically convert a hydrophilic to hydrophobic stateor provide functional groups such as amines, carboxylic acids, thiols orsulphonates to alter the surface characteristics of the substrate.Functionalization may include the optional primary step of oxidizing orashing the substrate in oxygen plasma to remove carbon and otherimpurities from the substrate surface and to provide a uniformlyreactive, oxidized surface which is then reacted with a silane. One suchpolymer that may be used is 3-aminopropyltriethoxysilane (APTS). Thesubstrate surface may be derivitized prior to application of a nanotubefabric.

Preferred embodiments use the nanotube-based switches of theincorporated, related references. As described therein, many volatileand non-volatile configurations may be used. These switches may then bearranged and sized as described above.

While single walled carbon nanotubes are preferred, multi-walled carbonnanotubes may be used. Also nanotubes may be used in conjunction withnanowires. Nanowires as mentioned herein is meant to mean singlenanowires, aggregates of non-woven nanowires, nanoclusters, nanowiresentangled with nanotubes comprising a nanofabric, mattes of nanowires,etc. The invention relates to the generation of nanoscopic conductiveelements used for any electronic application.

The following patent references refer to various techniques for creatingnanotube fabric articles and switches and are assigned to the assigneeof this application. Each is hereby incorporated by reference in theirentirety:

-   -   U.S. patent application Ser. No. 10/341,005, filed on Jan. 13,        2003, entitled Methods of making Carbon Nanotube Films, Layers,        Fabrics, Ribbons, Elements and Articles;    -   U.S. patent application Ser. No. 09/915,093, now U.S. Pat. No.        6,919,592, filed on Jul. 25, 2001, entitled Electromechanical        Memory Array Using Nanotube Ribbons and Method for Making Same;    -   U.S. patent application Ser. No. 10/033,032, now U.S. Pat. No.        6,784,028, filed on Dec. 28, 2001, entitled Methods of making        Electromechanical Three-Trace Junction Devices;    -   U.S. patent application Ser. No. 10/033,323, now U.S. Pat. No.        6,911,682, filed on Dec. 28, 2001, entitled Electromechanical        Three-Trace Junction Devices;    -   U.S. patent application Ser. No. 10/128,117, now U.S. Pat. No.        6,835,591, filed on Apr. 23, 2002, entitled Methods of NT Films        and Articles;    -   U.S. patent application Ser. No. 10/341,055, filed Jan. 13,        2003, entitled Methods of Using Thin Metal Layers to Make Carbon        Nanotube Films, Layers, Fabrics, Ribbons, Elements and Articles;    -   U.S. patent application Ser. No. 10/341,054, filed Jan. 13,        2003, entitled Methods of Using Pre-formed Nanotubes to Make        Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements and        Articles;    -   U.S. patent application Ser. No. 10/341,130, filed Jan. 13,        2003, entitled Carbon Nanotube Films, Layers, Fabrics, Ribbons,        Elements and Articles;    -   U.S. patent application Ser. No. 10/776,059, filed Feb. 11,        2004, entitled Devices Having Horizontally-Disposed Nanofabric        Articles and Methods of making The Same; and    -   U.S. patent application Ser. No. 10/776,572, now U.S. Pat. No.        6,924,538, filed Feb. 11, 2004, entitled Devices Having        Vertically-Disposed Nanofabric Articles and Methods of making        the Same.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of the equivalency ofthe claims are therefore intended to be embraced therein.

1. A driver for a load, the driver comprising: first and second inputnodes; a switching element, the switching element comprising: a controlelectrode in electrical communication with the first input node; arelease electrode in electrical communication with the second inputnode; an output node in electrical communication with the load; and ananotube element constructed and arranged to electrically contact theoutput node and thus the load in response to a control signal on thefirst input node, and to be out of electrical contact with the outputnode and thus the load in response to a release signal on the secondinput node, wherein, when the nanotube element electrically contacts theoutput node, one or more pre-selected electrical characteristics of theswitching element are sufficient to drive the load.
 2. The driver ofclaim 1, wherein the first and second input nodes carry differentialsignals.
 3. The driver of claim 1, wherein, when the nanotube elementelectrically contacts the output node, the nanotube element provides aconductive pathway between the control electrode and the output node. 4.The driver of claim 1, wherein the nanotube element is coupled toground.
 5. The driver of claim 1, wherein the nanotube element iscoupled to a supply voltage.
 6. The driver of claim 1, wherein the loadhas a capacitance of at least 10 pf.
 7. The driver of claim 1, whereinthe nanotube element comprises a porous nanotube fabric.
 8. The driverof claim 1, wherein the one or more pre-selected electricalcharacteristics comprises at least one of an impedance, a capacitance, aresistance, and a current-carrying capacity.
 9. The driver of claim 1,wherein the release electrode comprises an insulator layer on a surfaceof the release electrode facing the nanotube element.
 10. The driver ofclaim 1, wherein the load comprises a logic circuit.